(A) Field of the Invention
The present invention relates to a method for driving a light source and a backing light source, and more particularly, to a method for driving a light source and a backing light source with a power supply outputting quasi-constant current.
(B) Description of the Related Art
Light sources using light-emitting diodes (LEDs) possesses valuable characteristics such as high smoothness, high brilliance, mercury-free design, high color reproduction, space efficiency, etc.; therefore, higher value is added to electronic products (e.g., liquid crystal display units) using such light sources. As flat-screen television units have become increasingly popular among consumers and have gradually replaced CRT units as the market standard, manufactures in related fields seek a backing light source with preferred color representation. Since LEDs may achieve a brighter color gamma and have the advantage of a longer lifetime, these are the focus in the development of the flat-screen TV.
FIGS. 1 and 2 show conventional LEDs 16A, 16B and 16C, and a light source 10 using the same. In the light source 10, a pulse width modulation (PWM) signal generator 12 is used to generate turned-on signals, drivers (switching transistors) 18A, 18B and 18C are then conducted according to the turned-on signals such that a power supply 14 provides current through the blue LED 16A, the green LED 16B and the red LED 16C to emit blue light beams, green light beams and red light beams, which are combined into a white light. The turned-on signal generated by the PWM signal generator 12 is used to control the turned-on duration of the switching transistors 18A, 18B and 18C, and further controls the light-emitting time of the blue LED 16A, the green LED 16B and the red LED 16C.
Referring to FIG. 2, although the current flowing through the blue LED 16A, the green LED 16B and the red LED 16C are the same in magnitude, the power supply 14 must be switched three times within one cycle time, so that the output current of the power supply 14 is not a constant value. In general, the power supply 14 includes energy storage elements such as capacitors and inductors, and the switching operation makes the energy storage elements waste stored energy, resulting in an inefficient use of power output by the power supply 14.